1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus.
2. Description of Related Art
A fixing device of an image forming apparatus applies heat and pressure to a sheet, on which a toner image has been formed, with the sheet between a fixing roller and a pressure roller each having an elastic material layer on its surface. The fixing device thus fixes the toner image on the sheet.
The support shaft of the pressure roller of such a fixing device is rotatably supported by support arms, to which biasing members such as coil springs are attached. The biasing force of the biasing members allows the pressure roller to press the fixing roller at a given pressing force to maintain a constant nip width of the nip portion formed by the fixing roller and the pressure roller (see, for example, Japanese Unexamined Patent Application Publication Nos. 2006-259738 and 2003-177625).
In a process of producing conventional fixing devices, the biasing force of a biasing member is adjusted while the pressing force between a fixing roller and a pressure roller is measured, so that the nip width will be a given width.
The pressure roller and fixing roller of such a fixing device are detachable to be replaced when worn out.
If the hardness of a pressure roller etc. is the same before and after the replacement, the pressing forces to be obtained are the same before and after the replacement. In this case, a nip width can be maintained constant before and after the replacement. This enables maintenance of constant fixing properties and load torque of the roller, achieving uniform printing results.
Unfortunately, however, the hardness of a pressure roller etc. for replacement is not necessarily constant but varies from roller to roller. A change in hardness of a pressure roller etc. before and after the replacement results in a change in nip width. This leads to a change in fixing properties and load torque of the roller, making it difficult to achieve uniform printing results. In order to maintain the nip width at a given width, it is necessary to adjust the biasing force of a biasing member while measuring the pressing force each time a pressure roller etc. is replaced, which requires great workload.